Clutch fork stopper; it's to help prevent the TOB from dislodging from the pressure plate clip by over-returning.

 

is this a common problem on the evo's? first time i'm hearing of it

 

Not common, but also not unheard of. A bunch of us did/do run then on the 8/9 and it's starting to trickle over to the X since it's the same pull-style clutch design.

I started using one on my IX after I had a failure over 1,000 miles from home and somehow managed to limp the car home the entire way only make 3 stops for fuel.